With Boeing’s 737 Max variant in the news due to potential software issues that may have caused two fatal crashes, I was interested to read that software issues also caused the crash of the first Lockheed Martin/Sikorsky S-97 Raider prototype in 2017.
Nearly all helicopters have one main set of rotors. The S-97 has two sets of rotors, plus a propeller in the back intended to give it about twice the speed of traditional helicopters. In the 2017 crash, every rotor blade and every propeller blade either was bent or broken after the helicopter rolled more than 60 degrees to one side from a hover, and all parts of the landing gear collapsed when the pilot set it quickly back down.
The new National Transportation Safety Board’s factual report highlights the S-97′s fly-by-wire system, which uses electronic controls. When the helicopter is on the ground, its controls respond one way; when it’s been in the air for more than three seconds, the controls shift to a different mode. When it’s first taking off, controls transition between modes … “Control-law changes were introduced in late 2015 to improve ground-to-air transitions,” the NTSB reported. “An unintended byproduct of these changes was an increase of the cyclic stick (flight-control) sensitivity by 2.5 times while operating in the in-transition path.”
The pilot began taking off from a runway at Sikorsky’s plant off the Beeline Highway south of Indiantown Road. Nine-tenths of a second after the helicopter detected the weight was off its wheels, the helicopter began rolling a little to the left. The pilot tried rolling a little right, and “helicopter responds with a disproportionately large right roll,” the NTSB said. The pilot corrected left, and the left roll was “disproportionately large.”
Just one second after the pilot first touched the controls to counteract the minor roll, a wheel touched the ground again, resetting the transition to flight mode as the helicopter rolled again to the right. About a second and a half after that, the two sets of rotor blades were whipped enough around by the rolling that they struck each other.
All eight rotor blade tips came off. The pilot used the controls to bring the helicopter back down as fast as possible, and both the front and back landing gear collapsed. Every blade of the rear propeller was bent and showed signs that it had scraped the ground. The helicopter was resting on its belly, the NTSB reported.
There’s more at the link. Bold print is my emphasis.
Apparently the whole incident took less than five seconds from start to finish. Both pilots emerged from the crash with only minor injuries, and were able to get out of the aircraft and walk away without assistance, so it looks like the crash-protection features built into the fuselage worked as designed. That’s good . . . but it must still have been an “interesting” flight, to put it mildly!
It just goes to show how a software change can produce unintended consequences. The crashed aircraft was not rebuilt, but another prototype (with improved software!) took up the development process, and the S-97 has gone from strength to strength since then. It exceeded 200 mph in level flight late last year, a milestone for a helicopter of this size.
The S-97’s bigger brother, the Sikorsky-Boeing SB-1 Defiant, recently flew for the first time. If the coaxial rotor/propeller combination can be perfected, it’ll probably become a routine feature of future helicopters, and rival tiltrotor technology for dominance in the market.